feat: add wrapper for Middleware

[ArtARTs36]

This Pull Request adds wrapper from slog.Handler to slogx.Middleware

This will allow you to easy connect other popular extensions to slog e.g. slog-sentry, slog-nats and others that provide the slog.Handler interface.

Usage example:

package main

import (
	"os"

	slognats "github.com/samber/slog-nats"
	"github.com/cappuccinotm/slogx"
	"github.com/cappuccinotm/slogx/slogm"
	"log/slog"
)

func main() {
	h := slog.NewJSONHandler(os.Stderr, &slog.HandlerOptions{
		AddSource: true,
		Level:     slog.LevelInfo,
	})

	logger := slog.New(slogx.Accumulator(slogx.NewChain(h,
		slogm.RequestID(),
		// ...
		slogx.HandlerAsMiddleware(slognats.Option{Level: slog.LevelDebug, EncodedConnection: ec, Sub}.NewNATSHandler()),
	)))

	logger.Info("some message")
}

This will allow you not to connect additional libraries like slog-multi unless absolutely necessary.

[Semior001]

Thank you for your contribution! A few minor things

[Semior001]

LGTM